Ethanol is a viable, economical, and relatively clean fuel substitute or additive. It is commonly obtained from the fermentation of grain or other substances containing sugars and starches. Less commonly, cellulosic material obtained from waste sources can be converted to sugars, which can then be fermented to obtain ethanol. The use of such waste cellulose has been particularly attractive in the face of higher grain costs and concerns about waste disposal. The use of grain for the production of ethanol places a demand on grain, increasing its costs for alternative uses. By employing cellulosic waste materials in the production of ethanol, it is possible to reduce the demand for fossil fuels and grain-derived fuels.
Cellulosic material generally includes waste paper, agricultural chafe, municipal solid waste residual fluff, and wood products. Source material containing cellulosic material is typically obtained from municipal solid wastes, generally after the extraction of marketable goods. Source cellulosic material, however, can be obtained from any of a number of sources.
Cellulosic material can be converted to sugar via hydrolysis. Heretofore in the art, cellulosic material has been hydrolyzed by first reducing the material to a pulp and then reacting that pulp with sulfuric acid. Upon the introduction of heat, hydrolysis begins and the cellulosic material is converted to sugar. The reaction is quenched by rapid cooling of the mixture, followed by acid neutralization. Rapid quenching is necessary because the hydrolysis reaction is virtually instantaneous, and overexposure to heat and acidic conditions will result in the decomposition of the sugar product thereby reducing yield.
Numerous methods and reactions for carrying out hydrolysis are known in the art. For example, U.S. Pat. Nos. 3,853,759, 4,792,408, 5,711,817, and 5,879,637 disclose continuously flowing hydraulic columns wherein materials suspended in water are heated and gravity pressurized to effect hydrolysis. The heated material is forced upwardly through the column by column pressure and thereby cooled and depressurized. U.S. Pat. Nos. 5,711,817 and 5,879,637 disclose hydraulic columns (also termed “gravity pressure vessels”) for hydrolyzing cellulosic material, with means to control and manipulate the length of the hydrolysis reaction and control the quenching of the reaction. The present invention will be applicable particularly in the processes of those patents, but its application is not limited thereto or thereby.
The present invention is concerned mainly with the conversion of “woody” cellulosic material to useful end products through the use of a gravity pressure vessel and fermentation processes. Herein, “woody cellulosic material” is understood to refer to cellulosic materials containing a fibrous or woody matrix including variable percentages of heterogeneous components, including but not limited to: air pockets, lignin, glucan, moisture, xylan, ash or random metal oxides, uronic acids, arabinan, galactan, mannan, acetyl, soil, chlorophyll, proteins and other trace extracts as would be well known to those practiced in the arts. Though the gravity pressure vessel and related methods have been practiced with some success with other types of cellulosic material, difficulties have been encountered in the use of woody cellulosic material. Large amounts of energy are required to mechanically break down woody cellulosic material to expose the cellulosic material for the hydrolysis to sugars. The energy input to break the woody cellulosic material down to a size where free fibers can be converted to saccharides through weak acid hydrolysis can be significantly larger than the energy resulting from the ethanol produced from the cellulosic material. Additionally, air pockets in woody cellulosic materials can cause the materials to float in a carrier fluid through the gravity pressure vessel, preventing the materials from flowing through the system. As a result, such woody cellulosic materials either are not employed in the gravity pressure vessels or, to the extent that they might be so employed, are generally refractory to the existing arts.
The present invention seeks to satisfy the need in the art for a process for reducing the size of woody cellulosic materials fed to gravity pressure vessels, and seeks also to better suspend the woody cellulosic material in the carrier fluid of the gravity pressure vessel, in order to increase the efficiency of the process of converting the cellulosic material to ethanol via acid hydrolysis and fermentation.